Low density webs that are used to produce absorbent tissue products (e.g. facial tissues, bath tissues and other similar products) are designed to include several important properties. For example, it is desirable that the products have good bulk, a soft feel and absorbency. It is also desired that the product have good strength and resist tearing, even while wet. Unfortunately, it is very difficult to produce a high strength tissue product that is also soft and highly absorbent. Usually, when steps are taken to increase one property of the product, other characteristics of the product are adversely affected.
For instance, softness is typically increased by decreasing or reducing cellulosic fiber bonding within the tissue product. Inhibiting or reducing fiber bonding, however, adversely affects the strength of the tissue web.
Softness may be enhanced by the topical addition of a softening agent to the tissue web. For example, recently those skilled in the art have proposed applying aqueous dispersions containing polymer particles to tissue webs for increasing softness. Such polymer dispersions are disclosed, for instance, in U.S. Pat. No. 7,785,443, U.S. Pat. No. 7,820,010, U.S. Pat. No. 7,807,023 and U.S. Patent Application Publication No. 2008/0073045, which are all incorporated herein by reference. In the above patents and patent application, the aqueous dispersion contains an alpha-olefin polymer, an ethylene-carboxylic acid copolymer, or mixtures thereof. In addition to increasing softness, the polymer dispersion has been found to reduce lint, sheet to sheet adhesion, and even improve strength. In fact, the above patents and patent application represent great advances in the art.
Problems have been experienced, however, in applying the above polymer dispersions to tissue webs. More particularly, problems have been experienced in applying the polymer dispersions to tissue webs without having to crepe the webs. When applying the polymer dispersion without the assistance of a creping surface, the dispersion can either be applied to the web before drying while the web is still wet or after drying in a post-treatment stage. Unfortunately, if the dispersion is applied to a low density web in the above situations, it tends to penetrate the web which can increase the stiffness of the product. Thus, a need exists for a process for applying compositions, such as polymer dispersions, to base sheets such that the composition remains on the surface of the sheet in controlled amounts. A need also exists for a process capable of applying a composition to a surface of a base sheet at normal processing speeds.